[time-nuts] PTPv2 grandmaster with a Z3805A?

Wojciech Owczarek wojciech at owczarek.co.uk
Tue Aug 20 13:41:57 EDT 2013


On 20 August 2013 17:16, Mike S <mikes at flatsurface.com> wrote:

>
> By default, the tsc is calibrated at each boot, which means that timing
> will likely change (and ntp drift values will be off) each time you boot.
>
> There's a linux kernel command line option which will fix that and provide
> consistency between boots, something like  "clocksource=tsc
> tsc_khz=2410988". The exact value depends on how fast the processor is
> clocked.


True, this will help, and needless to say, dynamic CPU frequency etc. is a
no-no, and it's best to bind the (ntp) process to a single CPU core.
However I find it that the drift differences will be sub-1ppm across
reboots. In my case (data center) they are in fact sub-500ppb - calculated
with ptpd.

You always need some stabilisation time after reboot, especially that your
wall clock will likely get re-initialised from RTC. I think overcoming that
may sometimes take longer than re-calculating drift.

I pointed to TSC mostly because of the PTP GM side of the original poster's
query.

Regards
Wojciech

-

Wojciech Owczarek


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